Conventionally, as a recording medium for recording digital data, an optical recording medium represented by CD and DVD has widely been utilized. In the optical recording medium, in general, a reflective layer is formed in the inside regardless of its recording and reproduction system. Moreover, those having a dielectric protective layer formed adjacent to the reflective layer have often been used.
As such a reflective layer, in general, a thin film of gold, silver, aluminum, an alloy thereof, silicon, or the like is used. However, since the reflective layer using gold or silicon has a low reflectance with respect to a blue laser so that an optical recording medium using such a reflective layer cannot be used for an optical recording device using a blue laser for recording or reproducing the information. Moreover, since an optical recording medium having a reflective layer using aluminum or an alloy thereof has a relatively large crystal particle size of the aluminum or the like, in the case where it is used for an optical recording device using a blue laser, a problem is involved in that sufficient recording and reproducing characteristics cannot be obtained due to the large noise.
Moreover, although a reflective layer using silver or a silver alloy is compatible to a device using a blue laser, problems are involved in that its cost is high and that the silver is a control subject of the “Law Concerning Reporting, etc. of Releases to the Environment of Specific Chemical Substances and Promoting Improvements in Their Management” (hereinafter, it is also referred to as the PRTR law). Moreover, in general, a composition made of a zinc sulfide and a silicon dioxide is used for a dielectric protective layer to be formed in contact with the reflective layer so that in the case where silver is used for the reflective layer, it can easily be corroded by the sulfur included in the zinc sulfide in the dielectric protective layer, and thus it is also problematic.
Furthermore, as to the zinc in the zinc sulfide used as the material for the dielectric protective layer, it is a control subject of the PRTR law so that a problem of the concern to the influence on the environment is involved.
A prior art concerning the present invention is not found.